Casino operators strive to motivate gamblers to wager to maximize their gaming revenue. This motivation may be achieved by increasing gamblers’ perceived likelihood of winning, which stems from cognitive bias. Therefore, it is important for casino operators to understand the components that promote cognitive bias in gambling. This study introduces and demonstrates the existence of a bias of size in gambling decisions using a casino game referred to as Cussec. Specifically, participants are expected to bias their choice toward the option of Big (vs. Small) in the game; however, these two options are of equal chance. In the present research, a field observational study and two experimental studies were conducted. In the observational study, the bias was identified using 4,565 Cussec bets that were recorded in a casino in Macao. The bias was further demonstrated in experimental studies that involved Chinese participants (389 participants in the laboratory and 1,000 participants through an online survey platform). The bias is robust against the order of appearance (i.e., Big is displayed before or after Small). The online experiment contributes knowledge to the literature by indicating that gamblers’ bias toward Big distorts their perceived likelihood of winning. It is recommended that casino operators provide more games that feature size cues, and gaming designers incorporate these cues in the design of casino games.